To transmit an optical repeater supervisory signal in an optical amplifier repeater system to the end office side, there have been proposed the following systems which superimpose the supervisory signal on the main optical signal to such an extent as not to affect the main signal.
(1) System which superimposes the supervisory signal on the envelope of the main optical signal with a low modulation depth. (For example, Japanese Pat. Laid-Open No. 119328/90, "Optical Repeater Supervisory Control System")
(2) System which superimposes the supervisory signal on the main optical signal, using an optical signal of a wavelength different from that of the main optical signal.
The repeater supervisory signal is superimposed on the main optical signal and the both signals are received simultaneously; when the supervisory signal is received at the end office, the main signal is also received. Consequently, the signal spectrum of the supervisory signal after photoelectric conversion overlaps the power spectrum of the main signal, and since the supervisory signal is superimposed on the main signal at such an extremely low level as not to exert any influence on the main signal, the main signal component becomes noise when the supervisory signal is received; hence, it is difficult to receive the supervisory signal with a sufficient signal power vs. noise power ratio (SNR).
The inventors of this application has proposed a solution to this problem in Japanese Pat. Appln. No. 337902/92, entitled "Supervisory Signal Receiving Method and Apparatus." However, the method and apparatus proposed in the prior application are defective in that the function of keeping the main signal component at minimum at all times is somewhat poor.
An object of the present invention is to provide a supervisory signal receiving method and apparatus which ensure stable and reliable supervision of repeaters by receiving the repeater supervisory signal with a higher SN ratio in the optical amplifier repeater system.